1. Field of the Invention
The present invention relates to novel photosensitive materials having an improved protective layer.
2. Description of the Prior Art
The electrophotographic process generally comprises the steps of imposing a uniform positive or negative charge on a photoconductive layer formed on a conductive support, conducting imagewise exposure to thereby form an electrostatic latent image, developing the latent image with a developer prepared by dispersing toner particles in a proper carrier (gas, liquid or solid), and in the case of the indirect electrophotographic process, transferring the image formed on the surface of the photosensitive material onto a transfer material such as an ordinary paper, etc., and then fixing the transferred image by applying heat or by virtue of the self-fixability of the toner.
With this method, the photosensitive material, after transfer of image, has residual toner on the photoconductive surface. Therefore, the surface must be cleaned with a brush or the like before reusing the material. As a result, the photoconductive layer of the photosensitive material may become damaged.
This damage manifests itself as flaws in the transferred image, which tend to become more apparent with repeated uses of the photosensitive material.
To avoid this damage, the photosensitive material should be made more durable. One known method for dealing with the problem is to provide a protective layer on the photoconductive layer.
A wide variety of materials have been previously employed to form protective layers. These have included, for example, such resins as polystyrene, poly-n-butyl methacrylate, polyamide, polyester, polyurethane, polycarbonate, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, ethyl cellulose, nitrocellulose and cellulose acetate.
In indirect electrophotography, in order to enhance the concentration of the transferred image and to facilitate the cleaning of the residual toner left on the surface of the photosensitive material, the toner image on the surface of the photosensitive materal should be transferred as completely as possible to the transfer material such as ordinary paper, etc. That is, the transfer rate must be enhanced as much as possible.
The transfer rate can be expressed by the following equation: ##EQU1## wherein, D.sub.T is the image density or concentration of whole image on the surface of photosensitive material (the concentration of image on the photosensitive material prior to the transfer of an ordinary paper)
D.sub.R is the image density concentration of residual image on the photosensitive material (the concentration of image on the photosensitive material after the transfer to an ordinary paper)
Both D.sub.T and D.sub.R are measured with a photographic densitometer after the image on the photosensitive material has been transferred onto a transparent sheet having its one side coated with an adhesive.
D.sub.T - D.sub.R is regarded as corresponding to the concentration of image transferred to the transfer material. The concentration of image is proportional to the amount of toner transferred.
A number of attempts have been made with a view to enhancing the transfer rate. These have been directed mainly to the fields of improvements in the developer and the copying apparatus. While some progress has been made, the results are still not completely satisfactory.